Aluminum surfaces for technical lighting purposes

ABSTRACT

Reflector for technical lighting purposes, having a reflecting surface of aluminum and a protective, transparent, pore-free barrier layer of aluminum oxide produced by anodizing having a dielectric constant ε of 6 to 10.5 at 20° C., where the barrier layer is of thickness d that either satisfies the condition 
     a) for constructive interference: 
     
         d·n=k·λ/2±20 nm 
    
     or 
     b) for achieving a color-toned reflector surface: 
     
          k·λ/2+20 nm!&lt;d·n&lt; (k+1)·λ/2-20 nm 
    
     ! 
     or 
     c) for using as starting material to produce reflectors with LI/HI multi-layer coatings that increase reflectivity. 
     
         d·n=l·λ/4±20 nm 
    
     where n is the refractive index of the barrier layer, λ is the average wave length of the light striking the surface of the reflector, k is a natural number and l is a natural number that is uneven. The thickness of the barrier layer lies between 60 and 490 nm and does not vary by more than ±5% over the whole of the aluminum surface.

BACKGROUND OF THE INVENTION

The present invention relates to a reflector for technical lightingpurposes, having a surface of aluminum which is protected from physicaland chemical effects by a protective layer of aluminum oxide, andrelates also to its use and to a process for its manufacture.

Reflectors with brightened surfaces of high purity aluminum or AlMgalloys are known for the purpose of reflecting light in a directional ordiffuse manner. In order to achieve lasting brightness, the brightenedsurfaces are normally protected by an organic or inorganic coating or byan oxide layer. The oxide layers may be produced by chemical or anodicoxidation. Organic coatings may be produced by paint-type coatings,powder coatings or by laminating or coating with a plastic foil.Inorganic coatings may be made by PVD (physical vapor deposition), CVD(chemical vapor deposition), enamelling or plasma coating.

A widely used practice for manufacturing reflector surfaces is thedeposition of very thin, high purity PVD Al layers on glass; such layersare usually protected by a layer of e.g. PVD-Al₂ O₃, PVD-SiO₂ or apaint-type layer. Because of the thinness of the layer, PVD-Al layersgenerally cannot be anodized. Deposition of PVD-Al₂ O₃ or PVD-SiO₂layers is however expensive and, because of the homogeneity required toachieve good reflecting properties, the deposition of paint-type layersis complicated. Furthermore, paint-type layers generally exhibit onlymodest mechanical properties such as resistance to scratching, and oftenpoor stability with respect to UV-radiation.

Another protective layer often used today for reflector surfaces is madeby anodic oxidation utilizing direct current in a sulphuric acidelectrolyte. The resultant protective layer exhibits a uniform layerthickness but, as a result of the process itself, exhibits highporosity. Anodic oxidation in sulphuric acid electrolytes is normallycalled a dc process. To achieve sufficient reflectivity using thatmethod, the aluminum surfaces that are to serve as the reflectorsurfaces are normally brightened chemically or electrolytically, andsubsequently protected by a transparent protective layer e.g. by a dcprocess. The concentration of sulphuric acid in the dc process istypically 20 wt %, the electrolyte temperature 15° to 30° C., theapplied voltage 12 to 30V and the current density 1 to 3 A/dm². Thethickness of layer achieved is typically 1 to 10 μm; the layers obtainedare colorless to yellowish.

The oxide layer produced by the dc process is generally comprised of twolayers viz., a pore-free, very thin base or barrier layer and a porousouter layer. The pores are produced as a result of the oxide layer beingpartially re-dissolved, chemically, at the surface exposed to theelectrolyte. The total thickness of the oxide skin reaches its upperlimit when growth and dissolution are balanced, which depends on thecomposition of the electrolyte, the current density and the temperatureof the electrolyte.

In order to achieve adequate protection from corrosion, the porouslayers produced by the dc process have to be sealed. This is normallycarried out using boiling water (>96° C.) or water vapor (>98° C.).During this hydrothermal sealing the aluminum oxide swells as a resultof absorbing water and the pores are closed. In the process a part ofthe aluminum oxide is transformed to aluminum monohydrate.

On sealing in boiling water or steam, however, often an undesired,tightly adherent sealing deposit (so called smut) is formed. As a resultof atmospheric effects this smut leads to disturbing deposits which arematt to iridescent and lead to interference colors. For that reason thesealing deposits have to be removed by abrasive means. One possibilityfor preventing such sealing deposits is to employ special sealing baths.

The only anodic oxide layers produced in sulphuric acid that arecolorless and clear are those produced on high purity aluminum and AlMgor AlMgSi alloys with high purity aluminum (> or=99.85 wt % Al). In mostconstruction alloys, as a result of heterogeneous precipitates presentin the structure, more or less cloudy oxide layers are formed. Also, inmost alloys, if the heat treatment is unfavorable, precipitation occursin the structure leading to grey discoloration such as e.g. spots due tolocal thermal effects.

In the case of most protective surface layers produced on aluminum usingthe dc process, said layers for reflectors typically being 1 to 10 μmthick, and in particular in the case of less pure materials such as e.g.Al 99.85, Al 99.8 or Al 99.5, alloying elements such as e.g. Fe-rich orSi-rich intermetallic phases may be incorporated in the oxide layerleading to undesired absorption or scattering of light i.e. the light isreflected at various angles. As a result, the technical characteristicsof reflected light after the brightening treatment i.e. values such ase.g. the total reflectivity or the directional reflectivity, areinfluenced in a detrimental manner.

Due to the large thickness of the oxide layers produced by a andintegral to, the dc process, the reflectivity of the surface is reducedby the absorption and scattering of light. Finally, the oxide layer inthe normal thickness range of 1 to 3 μm often exhibits disturbinginterference effects, so called iridescence.

SUMMARY OF THE INVENTION

The object of the present invention is to avoid the above mentioneddisadvantages and to propose reflectors for technical lighting purposesthat possess at least one surface, or parts at least of one surface,that enable incident light to be reflected with as little loss ofreflectivity as possible.

That objective is achieved by way of the invention in that the reflectorfor technical lighting purposes has an aluminum surface and aprotective, transparent, pore-free barrier layer of aluminum oxideproduced by anodizing that exhibits a dielectric constant ε of 6 to 10.5at 20° C., where the barrier layer is of thickness d that eithersatisfies the following condition

a) for constructive interference:

    d·n=k·λ/2±20 nm

or

b) for achieving a color-toned reflector surface:

     k·λ/2+20 nm!<d·n< (k+1)·λ/2-20 nm!

or

c) for using as starting material to produce reflectors with LI/HImulti-layer coatings that increase reflectivity.

    d·n=l·λ/4±20 nm

where n is the refractive index of the barrier layer, λ is the averagewave length of the light striking the surface of the reflector, k is anatural number and l is a natural number that is uneven, the thicknessof the barrier layer lies between 60 and 490 nm and does not vary bymore than ±5% over the whole of the aluminum surface. At the same timeit must be taken into account that, because of dispersion, therefractive index n is dependent on the wave length i.e. in the presenttext n always refers to the corresponding wave length of the lightstriking the surface of the reflector.

The aluminum surfaces required for the reflectors according to theinvention may concern surfaces of aluminum parts, strips, sheets orfoil, also aluminum outer layers on items made from composite materials,in particular aluminum outer layers on composite panels, or aluminumlayers deposited--for example electrolytically--on any material. In apreferred version the item featuring the aluminum surface concerns aworkpiece of aluminum which has been manufactured e.g. by a rolling,extrusion, forging or impact extrusion methods.

When referring to the material aluminum in the following text, this isto be understood as including all grades of purity and all aluminumalloys. In particular the term aluminum includes all rolling, wrought,casting, forging and extrusion alloys of aluminum. The aluminum surfacesmay be of high purity aluminum alloys having a purity of 99.99 wt % Aland higher, for example a clad material, or exhibit a purity of 99.5 to99.99 wt % Al. The aluminum surfaces of the reflectors according to theinvention preferably exhibit a purity of less than 99.99 wt % Al, inparticular a purity of 99.5 to 99.98 wt % Al. Especially preferred arealuminum surfaces exhibiting a purity of 99.8 to 99.98 wt % Al.

The barrier layers on the reflectors according to the invention exhibiton aluminum surfaces of purity 99.5 to 99.98 wt % Al essentially nochanges in technical lighting characteristics compared to the surfaceproperties of the original aluminum surface, i.e. the condition of thesurface of the aluminum, as it was e.g. after brightening, is to a largeextent retained after the barrier layer is formed. To be taken intoaccount here, however, is that the metal purity of the surface layer canindeed have an influence on the resultant brightness, as a with respectto the brightening of aluminum surfaces, it is known that the lower thepurity of the aluminum, the poorer is the brightness obtained, and withthat also the poorer the reflective properties.

The aluminum surfaces may have any shape and may also be structured. Inthe case of rolled aluminum surfaces, these may be treated e.g. by highfinish or designer rolls. A preferred use of structured aluminumsurfaces is e.g. in reflectors used for daylight lighting, in particularstructured surfaces with structure sizes of 0.1 to 1 mm.

The barrier layer exhibits a constant thickness and does not vary bymore than ±5% over the whole of the aluminum surface. This makes itpossible for the first time to employ reflectors for almost loss-freereflection of electromagnetic waves in technical lighting applications,as for the first time, the reproducible, uniform thickness of thebarrier layer permits reflectors to be used e.g. for reflection withconstructive interference or to achieve exactly defined color tones.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a comparison of the directional reflectivity of abrightened aluminum surface with that of a brightened aluminum surfaceof the same purity bearing a 150 nm thick barrier layer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In order that, in the first place, constructive interference can occurand, secondly, the absorption of light in the barrier layer is as smallas possible, it is essential to the invention that a transparent barrierlayer is provided for the electromagnetic waves to be reflected. Afurther essential feature of the reflectors according to the inventionconcerns the absence of pores in the barrier layer.

The barrier layer must be pore-free in order that as little as possibleof the light penetrating the barrier layer is absorbed and any diffusescattering of the light which may arise due to the presence of pores,which is difficult to control, is also kept to a minimum. By the termpore-free is to be understood not absolutely pore-free, but rather thatthe barrier layer of the reflector according to the invention isessentially pore-free. Important in that respect is that the aluminumoxide layer produced by anodizing exhibits essentially no pores as aresult of the process, which means, no pores e.g. due to the use ofelectrolytes that dissolve aluminum oxide. In the case of the presentinvention the pore-free barrier layer in particular exhibits a porosityof less than 1%.

The dielectric constant E of the barrier layer depends, among otherthings, on the process parameters employed during anodizing to createthe barrier layer. In accordance with the invention the dielectricconstant ε of the barrier layer produced at 20° C. lies between 6 and10.5, preferably between 8 and 10.

In the case of reflectors according to the invention the thickness ofthe barrier layer is preferably chosen such that the reflector surfaceenables constructive interference of the reflected light to be achieved.The condition for constructive interference can be described asd·n=k·λ/2, where d·n is the optical layer thickness, n is the refractiveindex, λ is the average wave length of the light striking the reflectorsurface and k is a natural number. It must be noted that the conditionfor constructive interference described by the equation d·n=k·λ/2 isexactly valid only for light striking the reflector surfaceperpendicularly.

With regard to the thickness of the barrier layer, it was found in thecourse of the activities concerning the invention that the properties ofreflection run essentially periodically, viz., such that with increasinglayer thickness, in particular in the case of layers with optical layerthickness d·n greater than 3λ/2, the reflection properties areunsuitable for technical lighting purposes. Preferred therefore arelayers with a thickness that enable constructive interference of thereflected light to be achieved and k is a natural number, preferably 1,2 or 3, and 1 or 2 are particularly preferred. In the interest of thereflection properties the thickness of the barrier layer on reflectorsaccording to the invention preferably lie in the range of 60 to 490 nm(nanometers); especially preferred is the range 75 to 320 nm.

In the course of the work associated with the invention it was foundthat reflectors containing aluminum oxide surfaces with barrier layersof thickness that satisfies the condition for constructive interferenceand lie in the range d·n=k·λ/2±20 nm exhibit essentially the same goodreflection properties.

The refractive index n of the barrier layer usefully lies between 1.55and 1.65. Highly preferred is for the wavelength λ to correspond to theaverage wavelength that can be percepted best by the human eye duringdaylight, which lies approximately at 550 mm.

In a preferred version of the reflector according to the invention thebarrier layer is created such that it has a thickness d which fulfillsthe requirements for constructive interference d·n=k·λ/2±20 nm andabsorbs less than 3% of the incident light.

In the course of the activities concerning the invention it was foundthat barrier layers on brightened aluminum surfaces with a constant,uniform layer thickness over the whole of the reflector surface obtain auniform color tone, if the layer thickness d is chosen such that:

     k·λ/2±20 nm!<d·n< (k+1)·λ/2-20 nm!

In contrast to most layers containing colorant, these color tones arefast to light. Furthermore, it is possible to intensify or eliminate thecolor effects by using a polarization filter.

In a further version of the reflector according to the invention thebarrier layer is of thickness d according to the conditions d·n=l·λ/4±20nm where l is an uneven natural number. Such reflectors are suitable asstarting material for producing reflectors with reflection-improvingLI/HI-multilayers, i.e. LI/HI-multiple layers. By LI/HI-multilayers ismeant low/high-index-multilayers i.e. multiple layers made up of atleast two layers, with different refractive indices. The combination ofpairs of dielectric layers with different refractive indices on a metalsurface such that the layer with the lower refractive index is on themetal surface, enables an improvement in reflective properties comparedwith those of a single uniform layer. For a given layer composition, thehighest reflectivity can be achieved when the thickness of theindividual layers equals λ/4 or an uneven multiple thereof With regardto the composition of the layer material, the best reflection propertiesare obtained when the difference between the refractive indices of theindividual layers is as great as possible.

The thickness of the barrier layer according to the invention is smallin comparison to an oxide layer produced by dc anodizing. As a result,the former exhibits only few foreign particles that act as scatteringcenters for the light. Further, in the case of the reflectors accordingto the invention, the linearly dependent absorption of incident light issmall; as a result an increase in reflectivity is obtained over that ofknown reflectors with, for example, oxide layers produced by the dcprocess, especially if the thickness of the barrier layer is chosen suchthat constructive interference occurs. In comparison with oxide layersproduced by dc anodizing, barrier layers on reflectors according to theinvention are not affected by foreign particles such as e.g. particlesof Fe, Si, AlFeSi, i.e. such particles incorporated in the barrier layerdo not have a strong influence on light absorption that affects thetotal reflectivity, nor on the scattering of incident light.Furthermore, because of the small thickness of the barrier layer, theamount of light scattered at bent edges, especially those resulting fromcracks in the oxide layer, is normally negligibly small.

The normally large difference in layer thickness produced by the dcprocess causes selective light absorption and with that iridescenceproblems. On the other hand, the small and according to the inventionvery constant thickness of barrier layer over the reflector surface,causes no iridescence (rainbow colors). Furthermore, due to thethickness of the barrier layer, the spacing of the reflection planesleading to iridescence effects is too small. In the case of dc layers,the reflection plane spacings lie in the range of the wavelength of theincident light, with the result that iridescence can occur.

As a result of strong absorption by the oxide layer in the infra-redrange, reflectors bearing oxide layers made by dc anodizing partiallyexhibit only moderate reflectivity, which makes them unsuitable forinfra-red reflectors. In comparison, the reflectors according to theinvention exhibit no significant absorption of infra-red waves up to athickness of about 10 μm.

Also, if the reflectors according to the invention are subjected tofurther processing, they exhibit the significant advantage over known,state-of-the-art reflectors that e.g. on bending no visible cracks i.e.no shiny edges are formed.

The following table 1 shows a comparison of typical reflectivitycharacteristics, in particular the fractions of directionally andscattered refection values for a brightened aluminum surface without abarrier layer and brightened aluminum surfaces of different puritieshaving a 150 nm thick barrier layer. The brightened aluminum surfacewithout a barrier layer has a purity of 99.9 wt % Al. The brightenedaluminum surfaces of the reflectors with a barrier layer are of 99.50,99.9 and 99.98 wt % Al. The reflectivity values in table I were measuredaccording to the German Industrial Standard DIN 5036 and represent atechnical lighting characteristic i.e. the measured reflectivity valuesare weighted according to the sensitivity of the eye to light. As can beseen from the values obtained, the reflectivity is diminished only by aminimal amount by the barrier layer.

                  TABLE 1    ______________________________________                Total   Directional                                  Scattered                reflection                        reflection                                  reflection    ______________________________________    Al 99.9, brightened                  92.6%     88.3%     4.3%    Al 99.5%, brightened,                  88.3%     76.3%     12.0%    with barrier layer    Al 99.9, brightened,                  92.0%     87.4%     4.6%    with barrier layer    Al 99.8, brightened,                  92.0%     90.5%     1.5%    with barrier layer    ______________________________________

The present invention relates also to a process for producing reflectorsfor technical lighting purposes, featuring a surface of aluminum whichis protected by a transparent, pore-free barrier layer of giventhickness made of aluminum oxide produced by anodizing, having adielectric constant ε of 6 to 10.5 at 20° C. and varying by not morethan ±5% over the whole of the aluminum surface.

That object is achieved by way of the invention in that the aluminumsurface is oxidized electrolytically in an electrolyte that does notre-dissolve the aluminum oxide layer, and the desired thickness d of theresulting oxide layer, measured in nm (nanometers) is arrived at bychoosing and setting a constant electrolyte voltage U in volts selectedaccording to the following criteria

    d/1.4≦U≦d/1.2

where d/1.4 and d/1.2 represent quotients with d as the numerator and1.4 and 1.2 the denominator.

The process according to the invention permits thin, homogeneous and,with respect to electromagnetic radiation at least in the visible range,transparent barrier layers of uniform thickness to be manufactured. Whenlight in a medium such as e.g. air spreads out and enters another mediume.g. aluminum oxide in which the velocity of propagation of the light isdifferent (a different index of refraction), a fraction of the lightstriking the surface will be reflected back. Therefore, in order toachieve a reflector with uniform reflectivity characteristics over thewhole surface, it is necessary to have homogeneous layers of uniformthickness.

In the electrolytic process according to the invention at least thealuminum surface to be oxidized is provided with a previously specified,defined surface condition, then placed in an electrically conductivefluid, an electrolyte, and coupled up as the anode to a dc source wherethe negative electrode is normally stainless steel, graphite, lead oraluminum. According to the invention, the electrolyte is such that thealuminum oxide formed during the electrolytic process does not dissolvedchemically i.e. the aluminum oxide does not re-dissolve. In the dc fieldgaseous hydrogen is formed at the cathode and gaseous oxygen at theanode. The oxygen formed at the aluminum surface reacts with thealuminum so that the thickness of the oxide layer increases during theprocess. As the resistance of the layer increases rapidly withincreasing thickness of the barrier layer, the flow of current decreasesaccordingly and the layer stops growing.

The production of reflectors according to the invention requires a cleanaluminum surface, i.e. the surface that is to be oxidizedelectrolytically normally has to be subjected to surface treatment, a socalled pre-treatment, prior to the process according to the invention.

Aluminum surfaces normally exhibit an oxide layer that is formednaturally and, because of its previous history, is contaminated withforeign material. Such foreign material may e.g. be rolling lubricantresidues, protective oils for transportation purposes, corrosionproducts or pressed in foreign particles and the like. To remove suchforeign material, the aluminum surfaces are normally pre-treatedchemically with cleaning agents that attack the surface. Suitable forthat purpose, apart from acidic aqueous degreasing agents, are inparticular alkaline degreasing agents based on poly-phosphate andborate. Pickling or etching with strongly alkaline or acidic solutionssuch as e.g. caustic soda or a mixture of nitric acid and hydrofluoricacid cause moderate to pronounced attack and removal of material. As aresult, the natural oxide layer and all impurities incorporated in itare removed. Aggressive alkaline attack often produces deposits whichhave to be removed by an acidic after-treatment. Cleaning withoutremoval of surface material is effected by degreasing with organicsolvents or an aqueous or alkaline cleaning agent.

Depending on the state of the surface it may also be necessary to removethe surface by mechanical means viz., by abrasive materials. Such atreatment may be carried out e.g. by grinding, surface blasting,brushing or polishing, if necessary supplemented by a chemicalafter-treatment.

In the plain metal condition aluminum surfaces exhibit a very highcapacity for reflection of light and heat, whereby the smoother thesurface, the greater is the directional reflectivity and the shinier thesurface. Maximum brightness is achieved with high purity aluminum andspecial alloys such as e.g. AlMg or AlMgSi.

A highly reflecting surface is achieved e.g. by polishing, milling,rolling with highly polished rolls in the final roll pass, or bychemical or electrolytic brightening. Polishing may be accomplished e.g.using a buffing wheel of soft cloth and, if desired, employing apolishing paste. When polishing by rolling, it is possible in the lastroll pass also to impress a given structure into the aluminum surfacee.g. using engraved or etched steel rolls or a material bearing a givenstructure that is placed between the roll and the material being rolled.Chemical brightening takes place e.g. by using a highly concentratedacid mixture at high temperatures, normally over about 100° C. Acidic oralkaline electrolytes may be employed for electrolytic brightening;acidic electrolytes are normally preferred.

In order to maintain the bright finish, the brightened surface must beprotected from chemical and physical effects. The known state-of-the-artmethods for this, such as dc anodizing or coatings of paint exhibit theabove mentioned disadvantages such as large and difficult-to-controllayer thickness or inhomogeneous layers.

The process according to the invention provides homogeneous barrierlayers of uniform thickness that are essentially transparent in thevisible range so that the light can be reflected at the barrierlayer/aluminum interface.

Producing barrier layers electrolytically according to the process ofthe invention permits exact control over the thickness of the barrierlayer. The maximum thickness, in nanometers nm, of barrier layerobtained with the process according to the invention correspondsapproximately to the applied voltage in volts (V) i.e. the maximum layerthickness is a linear function of the voltage applied for anodizing. Theexact value of the maximum barrier layer thickness achieved as afunction of the applied voltage U can be determined by a simple trialand lies between 1.2 and 1.4 nm/V--the exact value of the thickness as afunction of the applied voltage being dependent on the electrolyte usedi.e. its composition and its temperature.

By employing an electrolyte that does not re-dissolve the barrier layer,these layers are almost pore-free i.e. any pores that occur are due e.g.to contaminants in the electrolyte or structural faults in the outeraluminum layer, but hardly due to the aluminum oxide dissolving in theelectrolyte. As the electrical resistivity of the oxide layer increasesmarkedly during the formation of the oxide layer, much higher voltagesare required than in the dc process.

Non-re-dissolving electrolytes that may be employed in the processaccording to the invention are e.g. organic or inorganic acids, as arule diluted with water and having a pH value of 2 and more, preferably3 and more, in particular 4 and more and 7 and less, preferably 6 andless, in particular 5.5 and less. Preferred are electrolytes that may beprocessed cold i.e. at room temperature. Especially preferred areinorganic or organic acids such as sulphuric or phosphoric acids at lowconcentrations, boric acid, adipinic acid, citric acid or tartaric acidor mixtures thereof, or solutions of ammonium or sodium salts of organicor inorganic acids, in particular the above mentioned acids and mixturesthereof. The said solutions exhibit a concentration in total of 20 g/lor less, usefully 2 to 15 g/l of ammonium or sodium salt dissolved inthe electrolyte. Very highly preferred are solutions of ammonium saltsof citric or tartaric acid or sodium salts of phosphoric acid.

A very highly preferred electrolyte contains 1 to 5 wt % tartaric acidto which e.g. an appropriate amount of ammonium hydroxide (NH₄ OH) maybe added to adjust the pH to the desired level.

As a rule the electrolytes are aqueous solutions.

The maximum possible anodizing voltage is determined by the dielectricvalue of the electrolyte. This depends for example on the compositionand temperature of the electrolyte and usually lies in the range of 300and 600V.

To increase the dielectric value of the electrolyte, alcohol may beadded as another solvent to it. Especially suitable for that purpose aremethanol, ethanol, propanol such as e.g. poly-propyl-alcohol oriso-propanol or butanol. The amount of alcohol to be added to theelectrolyte is not critical, so that the quantitative ratio ofelectrolyte to solvent may be e.g. 1:50. By adding alcohol thedielectric value of the electrolyte may be increased e.g. to 1200V. Forthe process according to the invention, however, alcohol-freeelectrolytes are preferred.

The optimum temperature for the process according to the inventiondepends on the electrolyte employed; in general, however, it is ofsecondary importance for the quality of the barrier layer obtained.Temperatures of 15° to 40° C., in particular temperatures between 18 and30° C., are preferred for the process according to the invention.

The process according to the invention is particularly suitable forproducing barrier layers continuously or on parts of coils, sheets,foils or items out of aluminum, and on composite materials having atleast an outer layer of aluminum. As such it has been found that usingaluminum of a purity equal to or greater than 99.5 wt % has nosignificant effect on the quality of the barrier layer i.e. the surfacecondition present after the brightening the aluminum surface remainsessentially the same after forming the barrier layer.

The process according to the invention is particularly suitable forelectrolytic oxidation of the aluminum surface in a continuous processemploying e.g. a treatment line, for example a strip anodizing process.

The process according to the invention exhibits the following advantagesover state-of the-art dc anodizing:

negligible consumption of electrolyte, as the aluminum oxide is notre-dissolved and the concentration of salts is normally very low (up toapprox. 20 g/l)

no sealing required

low consumption of electric current

In the dc process, as a result of the aluminum oxide re-dissolving, anenrichment of aluminum in the electrolyte takes place, whichcorrespondingly increases the consumption of electrolyte. Further, theelectrolyte for dc anodizing requires a high concentration of acid i.e.up to 200 g/l. In contrast to this, the concentration of salts in theelectrolyte for the process according to the invention is very smalli.e. up to approx. 20 g/l. Consequently the process according to theinvention results in only little incorporation of electrolyteconstituents in the oxide layer.

Producing a 2 m μm thick oxide layer by anodizing via the dc processresults in a current consumption of approx. 35,000 A/m², in contrast tothis, the consumption of current using the process according to theinvention to produce a typical barrier layer of 150 nm is only approx.2,500 A/m².

Producing oxide layers with the dc process requires very pure aluminumin order to prevent absorption or diffuse scattering of light by theinsoluble particles embedded in the thick oxide layer or to preventundesired iridescence due to interference effects; in the case of oxidelayers produced by the process according to the invention, by selectingthe appropriate thickness of barrier layer, such layers can be producedfree of iridescence even with less pure aluminum.

Reflectors according to the invention find preferential use in lamps fortechnical lighting purposes, especially in functional lamps e.g. forworkplaces with computer monitor screens, secondary lighting, spotlamps,or lighting elements such as light deflecting lamellae.

A preferred use for the reflectors according to the invention is fordecorative lamps especially color-tone lamps, or decorative surfacese.g. on ceiling or wall elements.

The reflectors according to the invention are also preferably employedfor producing color tones that are dependent on the angle ofillumination and/or angle of observation.

A further preferred use of the reflectors according to the invention isin their use as starting material for producing reflection intensifyingLI/HI multilayers. Required for that purpose are Al₂ O₃ barrier layersof thickness k×λ/4±20 nm, where λ is the average wavelength of the lightstriking the reflector surface and k is an uneven natural number.

Suitable HI-layers (high-index layer) for deposition on the barrierlayer according to the invention with an index of refraction of 1.6 aree.g. layers of titanium dioxide (TiO₂) with a refractive index ofapprox. 2.5, praseodymium--titanium-oxide (Pr--Ti-oxide),lanthanum--titanium-oxide (La--Ti-oxide), ZnS or CeO₂. Preferred,however, are HI-layers of TiO₂, Pr--Ti-oxide or La--Ti-oxide. TheHI-layers may be deposited e.g. using PVD methods or by decomposition oforganic compounds containing the desired metal oxides (e.g. via CVDmethods).

EXAMPLE

The aluminum surface employed was plain, rolled aluminum of 99.9 wt %purity which was subjected to the following pre-treatment:

1) Degreasing by boiling for 5 minutes

2) Rinsing

3) Neutralizing in HNO₃ (concentration HNO₃ :H₂ O=1:1)

4) Rinsing

5) Rinsing in H₂ O and deionised H₂ O

6) Immersion in a bath of ethanol

7) Drying with hot air

After the pre-treatment, the aluminum surface is brightened according tothe following procedure:

1) Immersion in a bath of cold electrolyte

2) Electrolyzed in H₃ PO₄ /H₂ SO₄ (specific gravity 1.755) for 60 sec.at 16V

3) Rinsing in H₂ O at 60° C.

4) Removal of the electrolyte deposit in NaOH (100 g/l) at 50° C. for 3sec.

5) Rinsing

6) Neutralizing in HNO₃ (concentration HNO₃ :H₂ O=1:1)

7) Rinsing in H₂ O and deionised H₂ O

8) Immersion in a bath of ethanol

9) Drying with hot air

The brightened aluminum surface was then anodized in citric acid at aconcentration of 1 g/l at room temperature; the applied voltage wasinitially 20V and this was increased continuously at a rate of 20 V/min.After increasing the voltage by 10 V the directional reflectivity of thealuminum surface bearing the barrier layer was measured each time at anangle of 20° to the surface normal using the method according to DIN67530 i.e. in comparison to an Al mirror, where the reflectivity of themirror represents 100%. The residual current, measured duringelectrolysis over the whole voltage range from 20 to 370 V, was lessthan 15 mA/dm.

FIG. 1 shows a comparison of the directional reflectivity of abrightened aluminum surface of 99.9 wt % Al with that of a brightenedaluminum surface of the same purity bearing a 150 nm thick barrierlayer. Plotted along the ordinate axis in FIG. 1 are the reflectivityvalues measured according to DIN 67530, and along the abscissa thevoltage applied for electrolysis. Curve a) shows the reflectivity valuesmeasured on the brightened aluminum surface, and curve b) shows thereflectivity values measured on the brightened aluminum surface bearingthe barrier layer. In each case the reflectivity values are given as the% of that of a standard mirror i.e. glass plate with aluminum vapordeposited on it and protected by a layer of SiO₂. Also shown in FIG. Iare the measured reflectivity values for barrier layer thicknesses atwhich the optical layer thickness amounts essentially to λ/4 (lambda/4)and essentially λ/2 (lambda/2).

I claim:
 1. Reflector for technical lighting purposes, which comprises areflector having a reflecting surface of aluminum and a protective,transparent, pore-free barrier layer of aluminum oxide produced byanodizing having a dielectric constant ε of 6 to 10.5 at 20° C., wherethe barrier layer is of thickness d that satisfies at least one of thefollowing conditions:a) for constructive interference:

    d·n=k·λ/2±20 nm,

b) for achieving a color-toned reflector surface:

     k·λ/2+20 nm! <d·n< (k+1)·λ/2-20 nm!, and

c) for using as starting material to produce reflectors with LI/HImulti-layer coatings that increase reflectivity:

    d·n=1·λ/4±20 nm,

where n is the refractive index of the barrier layer, λ is the averagewavelength of the light striking the surface of the reflector, k is anatural number and 1 is a natural number that is uneven, the thicknessof the barrier layer lies between 60 and 490 nm and does not vary bymore than ±5% over the whole of the aluminum surface.
 2. Reflectoraccording to claim 1, wherein the aluminum surface has a purity of 99.5to 99.98 wt. %.
 3. Reflector according to claim 1, wherein the thicknessof the barrier layer is between 75 and 320 nm.
 4. Reflector according toclaim 1, wherein λ corresponds to the average wavelength of light thatcan be best seen by the human eye during daylight.
 5. Reflectoraccording to claim 1, wherein the barrier layer has a thickness d thatsatisfies the condition for constructive interference:

    d·n=k·λ/2±20 nm,

and absorbs less than 3% of the energy of the incident light. 6.Reflectors according to claim 1, wherein the barrier layer exhibits athickness d that satisfies the condition:

    d·n=k λ/2±20 nm

where k is a natural number, for at least one of lamps in technicallighting applications, lamps in daylight, functional lamps forworkplaces with computer monitor screens, secondary lighting, spotlamps,light deflecting elements and light deflecting lamellae.
 7. Reflectorsaccording to claim 1, where the barrier layer exhibits a thickness d,that satisfies the condition:

     k·λ/2+20 nm!<d·n< (k+1)·λ/2-20 nm!,

where k is a natural number, for decorative lamps or the decorativesurfaces of ceiling or wall elements.
 8. Reflectors according to claim1, where the barrier layer exhibits a thickness d, that satisfies thecondition:

    d·n=1·λ/4±20 nm,

where 1 is an uneven natural number, as starting material formanufacturing LI/HI-multilayer barrier layers that increasereflectivity.
 9. Reflectors according to claim 1, where the barrierlayer exhibits a thickness d, that satisfies the condition:

     k·λ/2+20 nm!<d·n< (k+1)·λ/2-20 nm!,

where k is a natural number, for producing color tones that aredependent on at least one of the angle of illumination and the angle ofobservation.